Jamie Larsen is used to potato leafhoppers springing from the dry-bean plots at his home research station in Harrow, Ontario, and to the yellowing foliage, crisping edges and curling tips of "hopper burn". He was not expecting the same damage when he visited the Agriculture and Agri-Food Canada plots at Morden, Manitoba, this summer. Manitoba gets the occasional bad leafhopper year, but Larsen says it is easy to underestimate how much yield the insect takes. "It's kind of like a hidden yield robber," he said — even plants without obvious burn may be losing yield quietly.

The economic thresholds are low. At the first trifoliate, Ontario sets it at one insect per two plants; for older beans, Michigan State University material puts it at one insect per trifoliate leaf. "It's shocking, in fact, how low the thresholds are," Larsen said. At Harrow, where not all seed is treated, plots sometimes need several foliar passes a season and each brings a noticeable yield bump; he has also seen hopper burn in plots that a neonicotinoid seed treatment should have protected. The spraying needed to keep plots symptom-free is not viable on a working farm, he says, but it shows the yield at stake.

Manitoba Agriculture entomologist John Gavloski says agronomists take the pest seriously but it is hard to scout: "Potato leafhopper is a quite small insect; they're very mobile, so they're not as visible as some insects, and the damage, or hopper burn, that they result in, it can easily be confused with other things such as herbicide injury, nutrient deficiency, or even moisture stress." On the leaf their colouring is easily confused with aphids. The burn itself comes from an enzyme the insect injects while feeding, which blocks the plant's vascular system; it shows as a V-shaped patch that yellows, browns and may curl.

The leafhopper does not overwinter in Manitoba but migrates from the southern United States, often arriving on thunderstorms — sometimes in great numbers at once — and its short life cycle allows several generations in a summer. It thrives in dry conditions but a wet year does not rule it out, and alfalfa is a reservoir: "If alfalfa gets cut nearby a bean field, that could build the populations," Gavloski said. Agronomist Jason Voogt of Field 2 Field Agronomy sees it almost every year but rarely has to spray; this year leafhoppers have been noted since early June without large populations, and Gavloski has heard of no farms spraying.

Larsen's hope is genetics. Harrow now sees enough pressure to select more resistant cultivars, though resistance is complex — lower palatability, leaf structures that hinder the insect, or a colour it is less drawn to. Cutting the yield hit to 10% would, he said, be a win.

Source: Manitoba Co-operator